Previously, there have been many developments in the implantable drug delivery art. For instance, U.S. Pat. No. 4,594,058 discloses a single valve diaphragm pump that includes a pump housing, a flexible diaphragm reciprocally movable in the pump housing, and a filter means, and an outlet valve. The pump was designed to provide a stroke volume that is constant over various ranges of ambient pressure and reservoir pressure.
U.S. Pat. Nos. 5,067,943 and 5,088,983 discloses another plantable pump system which utilizes a flow regulator that isolates the regulator sensing chamber from the flow of the drug. The regulator uses either an independent sensing chamber or is coupled to a propellant chamber of the pump. Further, a downstream restrictor may be used to provide additional pressure drop between the regulator and the outlet chamber.